In the production of sulfuric acid by the contact process, the resulting SO.sub.3 is absorbed in sulfuric acid. In the normal catalytic process the SO.sub.2 -containing gas is catalytically treated (oxidized) in a plurality of series-connected catalyst trays, whereby the SO.sub.2 is largely converted to SO.sub.3, which is absorbed when it has left the catalytic reactor. In the double catalystic process, the SO.sub.3 formed in a first catalytic stage is absorbed in an interstage (intermediate) absorber. In the second catalytic stage, the remaining SO.sub.2 is converted to SO.sub.3 and when the SO.sub.3 formed in the second catalytic stage has left the catalytic reactor it is absorbed in the final absorber. A plurality of such interstage or intermediate absorbers may be provided.
During the absorption, the sulfuric acid is heated by the heat of reaction which is released and by the heat content of the SO.sub.3 -containing gases. Cooling is required to remove a corresponding quantity of heat from the acid.
In general, the heat is removed by cooling the absorption liquid (H.sub.2 SO.sub.4) in trickling coolers or air-cooled coolers. Owing to the small temperature rise of the cooling fluid, the quantity of heat which has been removed cannot be economically recovered and the heat is dissipated.
It is also known to cool the sulfuric acid in a pump receiver or to remove heat from an agitated sulfuric acid layer. In these processes too, a useful recovery of heat is not contemplated and complex and corrosion-sensitive apparatus must be used.
It is also known to cool the sulfuric acid effluent from the absorber in feed water preheaters and thus to heat the feed water. Because the temperature gradient is relatively low and the heat transfer is not large, the feed water preheaters must have large exchange surfaces. Also, the water requirement per cubic meter of acid is large.